Chronic back problems cause pain and disability for a large segment of the population. In many cases, chronic back problems are caused by intervertebral disc disease. When an intervertebral disc is diseased, the vertebrae between which the disc is positioned may be inadequately supported, resulting in persistent pain. Stabilization and/or arthrodesis of the intervertebral joint can reduce the pain and debilitating effects associated with disc disease.
Spinal stabilization systems and procedures have been developed to stabilize diseased intervertebral joints and, in some cases, to fuse the vertebrae that are adjacent to the diseased joint space. Most fusion techniques include removing some or all of the disc material from the affected joint, and stabilizing the joint by inserting an implant, for example, a bone graft or other material to facilitate fusion of the vertebrae, in the cleaned intervertebral space.
The use of bone grafts and bone substitute materials in orthopedic medicine is known. Conventionally, bone tissue regeneration is achieved by filling a bone repair site with a bone graft. Over time, the bone graft is incorporated by the host and new bone remodels the bone graft. In order to place the bone graft, it is common to use a monolithic bone graft or to form an osteoimplant comprising particulated bone in a carrier. The carrier is thus chosen to be biocompatible, to be resorbable, and to have release characteristics such that the bone graft is accessible. The natural cellular healing and remodeling mechanisms of the body coordinate removal of bone and bone grafts by osteoclast cells and formation of bone by osteoblast cells.
In the spinal surgery field, surgical procedures are often performed to correct problems with displaced, damaged or degenerated intervertebral discs due to trauma, disease or aging. Bone graft materials are often used in spine fusion surgery. Current spinal fusion implants utilize grafts of either bone or artificial implants to fill the intervertebral disc space.
In particular, one method of treating a damaged disc is by immobilizing the area around the injured portion and fusing the immobilized portion by promoting bone growth between the immobilized spine portions. This often requires implantation of an intervertebral device to provide the desired spacing between adjacent vertebrae to maintain foraminal height and decompression. That is, an intervertebral implant comprising an interbody fusion device may be inserted into the intervertebral disc space of two neighboring vertebral bodies or into the space created by removal of damaged portions of the spine.
In some instances, a formed implant, whether monolithic or particulated and in a carrier, is substantially solid at the time of implantation and thus does not conform to the implant site. Further, most implants are substantially formed at the time of implantation in limited sizes and shapes and provide little ability for customization.
While generally effective, the use of bone grafts has some limitations. Autologous bone grafts, being obtained from the patient, require additional surgery and present increased risks associated with its harvesting, such as risk of infection, blood loss and compromised structural integrity at the donor site. Bone grafts using cortical bone remodel slowly because of their limited porosity. Traditional bone substitute materials and bone chips are more quickly remodeled but cannot immediately provide mechanical support. In addition, while bone substitute materials and bone chips can be used to fill oddly shaped bone defects, such materials are not as well suited for wrapping or resurfacing bone. Indeed, the use of bone grafts is generally limited by the available shapes and sizes of grafts provided.
With regards to bone grafts, allograft bone is a reasonable bone graft substitute for autologous bone. It is readily available from cadavers and avoids the surgical complications and patient morbidity associated with harvesting autologous bone. Allograft bone is essentially a load-bearing matrix comprising cross-linked collagen, hydroxyapatite, and osteoinductive bone morphogenetic proteins. Human allograft tissue is widely used in orthopaedic surgery.
Indeed, an allograft implant is a preferred material by surgeons for conducting interbody fusions because it will remodel over time into host bone within the fusion mass. However, though allograft tissue has certain advantages over the other treatments, allograft implants are typically available in limited size ranges, thus making it difficult to provide implants, in particular, interbody implants in a preferred geometrical shape. Indeed, allograft implants may only provide temporary support, as it is difficult to manufacture the allograft with a consistent shape and strength. On the other hand, synthetic polymer implants such as poly-ether-ether-ketone (PEEK) can be manufactured into any geometrical shape. However, synthetic polymer implants, unlike allograft implants, have some strength limitations and will not remodel into host bone over time like an allograft implant. Synthetic polymer implants also do not allow for direct bone attachment or bonding to further stabilize the implant and fusion mass. In addition, surgical procedures are increasingly moving towards minimally invasive surgical procedures in which smaller interbody cages can be inserted through smaller surgical incisions and expanded once placed in the disc space. Because of the allograft size limitations, current expandable interbody cages are generally manufactured from metal and plastic materials.
Therefore, it would be desirable to construct an implant, particularly an interbody implant, that has components that allow remodeling and disc distraction.